Cobalt-nickel base alloys containing chromium and molybdenum

ABSTRACT

CORRISION-RESISTANT ALLOYS WHICH CAN BE WORK-STRENGTHENED TO HAVE A COMBINATION OF VERY HIGH ULTIMATE TENSILE STRENGTH, YIELD STRENGTH AND DUCTILITY, THE ALLOYS CONTAINING AS ESSENTIAL ELEMENTS, BY WEIGHT 33 TO 37% NICKEL, 7 TO 10.5% MOLYBDENUM, 19 TO 21% CHROMIUM, CARBON IN A MAXIMUM AMOUNT OF 0.025%, THE BALANCE BEING COBALT IN THE PROPORTION OF AT LEAST 33%, THE SUM OF COBALT AND NICKEL BEING FROM 66 TO 74% AND SAID SUM DIVIDED BY THE PERCENT CHROMIUM BEING FROM 3.1 TO 3.9.

United States Patent US. Cl. 148-115 Claims ABSTRACT OF THE DISCLOSURECorrosion-resistant alloys which can be work-strengthened to have acombination of very high ultimate tensile strength, yield strength andductility, the alloys containing as essential elements, by weight, 33 to37% nickel, 7 to 10.5% molybdenum, 19 to 21% chromium, carbon in amaximum amount of 0.025%, the balance being cohalt in the proportion ofat least 33%, the sum of cobalt and nickel being from 66 to 74% and saidsum divided by the percent chromium being from 3.1 to 3.9.

CROSS REFERENCES TO RELATED APPLICATIONS This application is acontinuation-in-part of my copending application Ser. No. 637,613, filedApr. 10, 1967, now Pat. No. 3,356,542, as a continuation-in-part of mythen copending application Ser. Nov 584,029, filed Aug. 18, 1966, nowabandoned, and also as a continuation-inpart of my then copendingapplication Ser. No. 565,088, filed July 14, 1966, now abandoned, bothof said prior applications being filed as continuations-in-part of mythen copending application Ser. No. 452,054, filed Apr. 30, 1965, nowabandoned.

BACKGROUND OF THE INVENTION This invention is in the field ofnickel-base and cobaltbase alloys, particularly such alloys containingchromium and molybdenum and a critically small amount of carbon.

Manufacturing and processing conditions and procedures in chemical,mechanical, and metallurgical operations have advanced in recent yearsto the point Where the currently-used materials for the construction ofequipment have inadequate strength and resistance to oxidation andcorrosion.

Prior to the invention of my above-identified application Ser. No.637,613, now Pat. No. 3,356,542, cobaltand nickel-base alloys had nothad the combination of ductility and high yield strength or ultimatetensile strength desired for the above-noted and other high servicerequirements. Cold-work strengthening has generally been accompanied byexcessive loss of ductility; conversely, recrystallization or annealingto improve ductility has been accompanied by loss of strength derivedfrom working. In many prior art alloys the lack of ductility is so greatas to amount to brittleness, but it had not been evident how' suchbrittleness could be avoided while retaining very high or enhancedstrengths.

There had also been a need for ductile, strong alloys of the type justdescribed which, additionally, had a high degree of resistance tocorrosion under stress, especially in sea water. Stress-corrosion maylead to sudden, pre mature failure in such marine hardware as wire andcable. Obviously, failure without warning is incompatible with marinesafety.

According to the invention of my prior application Ser. No. 637,613, nowPat. No. 3,356,542, the above-discussed 3,562,024 Patented Feb. 9, 1971problems of the prior art were solved by the thereindescribed novelalloys which in their broadest aspects were strong and adequatelyductile, and were highly resistant to stress-corrosion in sea water.These alloys consist essentially of, by weight, 5 to 45% nickel, 7 to16% molybdenum, 13 to 25% chromium, up to 0.05% carbon, up to 2%aluminum, up to 2% zirconium, the sum of aluminum, titanium andzirconium being no greater than 4%, up to 0.5% silicon, up to 6% copper,up to 6% iron, and incidental elements and not exceeding 0.1%, thebalance being cobalt in the proportion of at least 25%, the sum of thecobalt and nickel being in the range of 62 to 80% and the sum of thepercents of cobalt and nickel divided by the percent chromium being atleast 2.6. Preferably the nickel is 5 to 40%, the sum of nickel andcobalt is 65 to the molybdenum is 8 to 14%, and the chromium is 15 to22%.

SUMMARY Now according to the present invention it has been found thatwithin the scope of alloy compositions described in my prior applicationSer. No. 637,613 new Pat. No. 3,356,542, there is a limited combinationof elements which give novel alloy compositions capable of beingwork-strengthened to products having an unexpectedly optimum combinationof high tensile strength, high yield strength, and ductility. The novelcompositions are corrosion resistant. They consist essentially of, byweight, 33 to 37% nickel, 7 to 10.5% molybdenum, 19 to 21% chromium,carbon in a maximum amount of 0.025%, the balance being cobalt in theproportion of at least 33%, the sum of cobalt and nickel being from 66to 74% and said sum divided by the percent chromium being from 3.1 to3.9.

In a preferred aspect, alloys of the above-stated composition arework-strengthened to give alloy products which retain their resistanceto stress-corrosion in sea water and which consist essentially of amatrix phase having dispersed therein at least 5 volume percent of aplatelet phase, said matrix phase being a solid solution of the alloycomposition having a face-centered-cubic crystal structure and saidplatelet phase being a solid solution of the alloy composition having ahexagonalclose-packed crystal structure, the platelets being about from20 to 1000 A. thick and being distributed substantially on the [111]planes of the matrix phase crystals, said work-strengthened alloy beingcharacterized by having, in a tensile test at 68 F an ultimate tensilestrength of greater than 260,000 p.s.i., a 0.2% yield strength ofgreater than 240,000 p.s.i., and a ductility, as measured by reductionin area, of greater than 40%.

By working a body of the alloy composition, at a temperature between itsmelting temperature and the upper temperature of its transformation zonebut at least about 1200 F., until its cross-sectional area has beenreduced by at least 5 an intermediate material is obtained which iscapable of being work-strengthened. Another intermediate material,similarly work-strengthenable but having maximum ductility, is preparedby heat-treating a body of the alloy at a temperature above 1950 F.until it is substantially homogenized, whereby the body has sufficientductility that its cross-sectional area can be reduced at least 40% atabout 68 F. Rather than prepare the intermediate alloy andwork-strengthen it, one may workstrengthen a body of the alloycomposition directly by working it at a temperature below the uppertemperature of its transformation zone until its cross-sectional areahas been reduced at least 5%, preferably 10 to DESCRIPTION OF THEPREFERRED EMBODIMENTS Novel alloy compositions of the invention can bemade by melting the component element metals together at a suitabletemperature, as in the range of 2300 to 3300 F., casting the molten massand cooling to form a solid-state body. Alternatively, the molten masscan be atomized and a solid-state, consolidated body can be formed byshaping, pressing and sintering the powder so obtained. The body formedby either of these methods can be workstrengthened, by working, at atemperature below the upper temperature of the transformation zone, toat least 5% reduction of cross-sectional area.

The novel products obtained by the work-strengthening processes arealloys consisting essentially of two phases: a matrix hase and at least5 volume percent and as high as 70 volume percent or higher, preferably550 volume percent, of a second phase of fine platelets, the matrixphase being a solid solution of the alloy components having aface-centered-cubic (fcc) crystal structure, and the platelets being asolid solution of the alloy components having a hexagonal-close-packed(hcp) crystal structure, the platelets being distributed on the [111]planes of the crystals of the matrix phase. The platelets are from threeatom layers (approximately 4.15 angstrom units) to 2500 angstrom unitsthick, preferably -1000 angstrom units thick, their width and lengthbeing at least 5 times, and as high as 10,000 times, their thickness;and the platelets are separated from one another by a distance of about10010,000 angstrom units, but preferably no greater than about 5,000angstrom units, i.e., one-half micron. Many of the platelets displaytwinning within their hexagonalclose-packed structure, the principalslip planes of the twins being approximately at right angles to theplanes of the untwinned regions.

It is believed that the strain-induced formation of the platelets,induced during the working step, within the facecentered-cubic structureof the metastable matrix phase is responsible for the importantimprovements in the properties of the products of this invention.Specifically, it has been discovered that within and below a limitingtemperature transformation zone, the face-centered-cubic structure(which exists at temperatures above the zone) can, by the definedworking step, be caused to transform into the strength-producinghexagonal-close-packed crystal form.

The products of this invention resulting from this described workingstep, have unusually high strength properties at room temperature.Specifically, their ultimate tensile strength is at least 260,000 p.s.i.and ranges to about 350,000 p.s.i. Their 0.2% yield strength is greaterthan 240,00 p.s.i. Their ductility, as defined by reduction in area, isat least 40% and may be as high as 75% or even higher. It should benoted that the strength properties of the products of this invention arefrom 3 to 4 or more times the strength properties of the alloy materialas cast. Furthermore, these products will retain these strengthproperties to a substantial extent after being subjected to temperaturesas high as 1200 F. for 100 hours. Some products within the scope of theclaims will substantially retain such properties after 100 hoursexposure to temperatures as high as 1400 F.

Products of the invention are extremely useful as fasteners, wire, andcable, and as dies for extruding such metals as aluminum and brass.Specifically, their hot strength and their hot strength retention willpermit the extrusion in quantity of red brass, heretofore impossiblewith conventional hot-worked tool steel dies. Furthermore, theseproducts are sufiiciently tough to serve as port or bridge dies foraluminum extrusion where sharp corners and inadequately supportedtongues require low notch sensitivity and the ability to deflect withoutfracturing. The products of this invention, being resistant to thermalstress, oxidation, and certain corrosive media, such as caustic, saline,and acid solutions, are suitable for use as such structural componentsas holders, backers, extrusion press liners and rams in extrusion; hotforging and coining dies; hot metal shears; hot metal swaging dies; andsimilar hardware. The intermediate alloy products hereindescribed, evenbefore work-strengthening, have unique resistance to stress-corrosion,especially in sea water, and are suitable for use in marine hardware.

In the preparation of these products, it is especially convenient to usecommercially pure elements. Since only minor changes in the relativeproportions of the essential elements will occur during the processing,it is possible to start with amounts of the components that are desiredin the final product. Thus, these amounts are melted in a furnacedesigned for melting alloys in a temperature range of 23003300 F. andthe resulting molten composition is cast in molds or crucibles ofgraphite, cast iron, copper or ceramics. The composition may be meltcast in air, vacuum or in an inert atmosphere. Conventional shell andinvestment molds may be used for casting the shaped objects.

In a specific process, the elemental composition is melted in anopen-air induction furnace lined with magnesium oxide or silicon dioxideand is cast into cast iron molds. Initially, the desired amounts ofcobalt and nickel are melted, after which the molybdenum and chromiumare added. If vacuum melting is used, carbon is preferably employed todeoxidize the molten metal; any silicon which is to act as a deoxidantis added just prior to pouring the alloy composition. The conventionalhot-topping compounds may be used to minimize porosity and pipe in thearticles cast.

The amounts of chromium, molybdenum and cobalt, are critical inobtaining the desired final product. Thus, if the percentages of cobalt,chromium and/or molybdenum are above the maxima stated, the materialwill be too brittle to work at temperatures within and below thetransformation zone. If the percentages of molybdenum and/or chromiumare below the stated minima, the alloy will not respond adequately tothe work-strengthening step. In short, to respond to thework-strengthening step and produce the novel end products the alloymust contain the named components in the proportions specified.

It is critically important that the alloy composition contain, even asimpurities, no more than 0.05% of boron, oxygen, nitrogen or beryllium,and that the carbon content be below 0.025%, the total of thesecomponents being no more than 0.1%. Amounts greater than this,particularly amounts of carbon greater than that specified, will causesuch embrittlement as to make the work-strengthening non-operable. It isparticularly preferred to maintain the carbon content below 0.015% toinsure adequate workability and in an amount effective to provide atleast some carbide precipitation upon heat treatment.

In the powder metallurgy method of preparing the consolidated alloy bodyone first premelts the components together, then converts the resultingalloy to a powder, and finally converts the powder to the desired shapedarticle. The premelting step may involve arc melting and inductionmelting. The molten composition can be atomized to form the particulatematerial. The particle size of the powders can be further reduced bygrinding in steel or tungsten carbide-lined equipment. The resultingpowders can be readily shaped by cold pressing in steel dies atpressures ranging from about 10 to 50 tons per square inch. It ispreferred to sinter the cold-pressed objects at temperatures between1800 F. and 2500 F. for a period of 15 minutes to six hours in thepresence of an inert gas or hydrogen or in a vacuum furnace atmosphere.The powders can also be hot pressed in graphite dies at temperaturesbetween 2000 F. and 2400 F. using pressures of 1000 pounds per squareinch or higher,

After the alloy body has been consolidated by casting, pressing orotherwise, the material is Work-strengthened. The work-strengtheningprocess involves reducing the cross-sectional area of the article by atleast 5%, pref-- erably not more than 90%, more preferably 10% to at atemperature below the upper temperature of the transformation zone. Toreduce the area or to deform the body, any of the conventional metalworking techniques can be employed. Such techniques include forging,swaging, extruding, rolling, tube reducing, coining, drawing, pressing,explosive treatment, and impact loading. A convenient method ofdeforming the article is the swaging process. The swaging machine unitmay be a two-hammer, 30 horsepower machine with each set of swaging diesproducing a 12-20% reduction in area.

Although the work-strengthening step can be performed directly on thearticle as cast or pressed, it is preferred to at least partiallyhomogenize the cast alloy prior to the work-strengthening step, byheating the shaped article to a temperature which is between the uppertemperature of the transformation zone and the melting temperature. Thisheating step may be accompanied by working the article while it ismaintained at a temperature within the aforementioned range by swaging,rolling, forging, extruding, etc. to the extent of reducing itscrosssectional area by at least preferably 80%. The product, after thisheating step and prior to the workstrengthening step, is quite ductile,having an elongation of 4080%. This intermediate heating step makes iteasier to attain yield strengths of at least 240,000 p.s.i. (measured at0.2% olfset), and ultimate tensile strength greater than 260,000 p.s.i.,after the work-strengthening step.

The transformation zone is dependent to some extent on the particularalloy used and ranges from about 850- 925 F. minimum to about 11751250F. maximum for alloys of this invention. Therefore, it is apparent thatthe work-strengthening step, which must be performed below the uppertemperature of the transformation zone, preferably below thetransformation zone, can safely be performed on the specifiedcompositions at temperatures below 850 F. to achieve the results of thisinvention. However, higher temperatures that are still below the uppertemperature of the transformation zone will permit greater reductions inarea for any specified applied force. Hence, it may be desirable towork-strengthen at elevated temperatures. In fact, working can startwhile the material is at a temperature above the transformation zone andis being permitted to cool, provided at least 5% area reduction occurswhile the temperature of the material is below the upper temperature ofthe transformation zone. It should be understood that it is not alwayspossible to reduce the cross-sectional area up to 90% for all thecompositions falling within the specified range at a particulartemperature, particularly at the lower temperatures. For the purposes ofthis invention, workstrengthening to accomplish an area reduction of atleast 5% is critical.

As mentioned previously, the final work-strengthened product consistsessentially of two phases, an fcc matrix phase and 5-70 volume percentof the second phase of fine hcp platelets, the second phase beingdistributed on the [111] planes of the crystals of the matrix phase. Todetermine the presence of these phases, the amounts thereof, and theirlocation, techniques known to metallurgists and described inTransmission Electron Microscopy of Metals, G. Thomas (1962), JohnWiley, New York, may be used.

The presence of hcp phase is detected, for example, by the extremelysensitive techniques of electron transmission microscopy and electrontransmission diffraction. These techniques can be used to detect thestrain-induced formation in all alloys covered by this invention.Specimens can be electrolytically thinned to permit transmission of theelectron beam by the window method as described by G. Thomas on pp.153-155 of his aboveidentified book or by the jet cupping methoddescribed by P. R. Strutt.

Electron transmission micrographs and electron transmission diifractionpatterns were taken of small regions in the center of grains of severelydeformed alloys. The electron transmission diflFraction patternsconsisted of the P, R. ls crutt, Res. Sd. Inst. 32, 411, 1961.

single crystal pattern for the fcc grain plus the single crystal patternfor the hcp platelets on one or more of the four sets of [111] planes ofthe fcc grain. The diffraction patterns also indicated the presence ofmechanical twins within many of the hcp platelets, the principalslipplanes of the twins being approximately at right angles to theprincipal slip planes of the untwinned regions.

Measurements of the thickness of the hcp platelets and of the averagedistance of separation between platelets were obtained from the electrontransmission micrographs. The volume percent of the hcp phase formed bythe strain-induced transformation was determined by areal analysis ofthe electron transmission micrographs. For example, an alloy containing34.5% C0, 35% Ni, 20% Cr, 10% Mo, and 0.5% Si was rolled at roomtemperature. Areal analysis of electron micrographs of a sample fromthis alloy indicated the presence of approximately 30 volume percent hcpphase. The average thick ness of the hcp platelets was 300 angstromunits, and the average distance of separation between platelets was 1400angstrom units. The work-strengthened plural phase alloys of thisinvention are found to contain from 5 to 70 volume percent of hcpplatelets.

The following illustrative example constitutes a specific embodiment ofthe processes of this invention and is not intended to be limitative. Inthis example, various property data are reported. The test methods bywhich these data are obtained are, unless otherwise stated, the standardASTM test methods using standard ASTM specimens.

EXAMPLE lybdenum, 19.7% chromium, 0.04% silicon, 0.018% carbon, balancecobalt plus minor impurities. This heat was vacuum-melted and cast intoingots of varying weights and sizes. One of the ingots, 8" diameter by320 pounds, was turned to 7.6" diameter by 19" length, canned in a mildsteel jacket of /8" thickness, and extruded to 1.9" diameter using aningot preheat temperature of 2300 F. The extruded product was cropped,sandblasted, and pickled to yield sound bar conditioned for swaging.

The bar was swaged to 1.08" diameter with 2-hour anneals at 2,000 F. at1.45" diameter and 1.08 diameter. The bar was then pointed, lubricated,and drawn 49.5% reduction in area to 0.77" diameter. The resultant barwas aged for 4 hours at 900 F. The tensile and ductility properties wereas follows:

282,500 p.s.i. ultimate tensile strength 272,300 p.s.i. 0.2% yieldstrength 10.4% elongation 48.5% reduction in cross-sectional area.

I claim:

1. In a process for producing an intermediate material capable of beingwork-strengthened to give a ductile, high-strength alloy, the stepscomprising (1) preparing a body of metal consisting essentially of, byweight, 33 to 37 percent nickel, 7 to 10.5 percent molybdenum, 19 to 21percent chromium, carbon in a maximum amount of 0.025 percent, thebalance being cobalt in the proportion of at least 33 percent, the sumof the cobalt and nickel being in the range of 66 to 74 percent and thesum of the percents of cobalt and nickel divided by the percent chromiumbeing from 3.1 to 3.9, and (2) thereafter working said body at atemperature between its melting temperature and the upper temperature ofits transformation zone, said temperature being at least about 1175 F.to 1250 F., until its cross-sectional area has been reduced at least 5percent.

2. In a process for producing an intermediate material capable of beingwork-strengthened to give a ductile, highstrength alloy, the stepscomprising (1) preparing a body of metal consisting essentially of, byweight, 33 to 37 7 percent nickel, 7 to 10.5 percent molybdenum, 19 to21 percent chromium, carbon in a maximum amount of 0.025 percent, thebalance being cobalt in the proportion of at least 33 percent, the sumof the cobalt and nickel being in the range of 66 to 74 percent and thesum of the percents of cobalt and nickel divided by the percent chromiumbeing from 3.1 to 3.9, and (2) heat-treating said body at a temperatureabove 1950 F. until it is substantially homogenized, whereby the bodyhas sufiicient ductility that its cross-sectional area can be reduced atleast 40 percent at about 68 F.

3. In a process for producing a work-strengthened alloy, the stepscomprising (1) preparing a body of metal consisting essentially of, byweight, 33 to 37 percent nickel, 7 to 10.5 percent molybdenum, 19 to 21percent chromium, carbon in a maximum amount of 0.025 percent, thebalance being cobalt in the proportion of at least 33 percent, the sumof the cobalt and nickel being in the range of 66 to 74 percent and thesum of the percents of cobalt and nickel divided by the percent chromiumbeing from 3.1 to 3.9, and (2) thereafter working said body at atemperature below the upper temperature of its transformation zone untilits cross-sectional area has been reduced at least 5 percent.

4. An alloy body consisting essentially of, by weight, 33 to 37 percentnickel, 7 to 10.5 percent molybdenum, 19 to 21 percent chromium, carbonin a maximum amount of 0.025 percent, the balance being cobalt in theproportion of at least 33 percent, the sum of the cobalt and nickelbeing in the range of 66 to 74 percent and the sum of the percents ofcobalt and nickel divided by the percent chromium being from 3.1 to 3.9,said body having been worked at a temperature between its meltingtemperature and the upper temperature of its transformation zone, saidtemperature being at least 1175 F., until its cross-sectional area hasbeen reduced by at least 5 percent, and being further characterized bycontaining substantially no hexagonal close-packed phase.

5. An alloy body as in claim 4, wherein carbon is present in an amountless than 0.015 percent.

6. An alloy body consisting essentially of, by weight, 33 to 37 percentnickel, 7 to 10.5 percent molybdenum, 19 to 21 percent chromium, carbonin a maximum amount of 0.025 percent, the balance being cobalt in theproportion of at least 33 percent, the sum of cobalt and nickel being inthe range of 66 to 74 percent and the sum of the percents of cobalt andnickel divided by the percent chromium being from 3.1 to 3.9, said bodyhaving been heat-treated above 1950 F. until substantially homogeneous,and being characterized by having sufficient ductility that itscross-sectional area can be reduced at least 40 percent at about 68 F.,and by containing substantially no hexagonal close-packed phase.

7. An alloy body as in claim 6 wherein carbon is present in an amountless than 0.015 percent.

8. A work-strengthened alloy consisting essentially of a matrix phasehaving dispersed therein at least 5 volume percent of a platelet phase,said matrix phase having a face-centered-cubic crystal structure andbeing a solid solution of an alloy consisting essentially of, by weight,33 to 37 percent nickel, 7 to 10.5 percent molybdenum, 19 to 21 percentchromium, carbon in a maximum amount of 0.025 percent, the balance beingcobalt in the proportion of at least 33 percent, the sum of the cobaltand nickel being in the range of 66 to 74 percent and the sum of thepercents of cobalt and nickel divided by the percent chromium being from3.1 to 3.9, said platelet phase being a solid solution of the same alloycomposition but having a hexagonal-closepacked crystal structure, theplatelets being about from 20 to 1000 A. thick and being distributedsubstantially on the [111] planes of the matrix-phase crystals, saidwork-strengthened alloy being characterized by having, in a tensile testat 68 R, an ultimate tensile greater than 260,000 p.s.i., a 0.2 percentyield strength of greater than 240,000 p.s.i., and a ductility, asmeasured by reduction in area, greater than 40 percent.

9. A work-strengthened alloy as in claim 8 which has been aged at least0.5 hours at 600 F. to 1200 F.

10. A work-strengthened alloy as in claim 8 wherein carbon is present insaid alloy composition in an amount less than 0.015 percent.

References Cited UNITED STATES PATENTS 2,247,643 7/1941 Rohn et al. -1712,712,498 7/1955 Gresham et al 7517l 3,356,542 12/1967 Smith 1482RICHARD O. DEAN, Primary Examiner US. Cl. X.R.

148l2.7, 13, 32, 32.5; 7517l; 29l82

